Linearizers can be used in broadband communication equipment where linearization is desired over a wide frequency range. The linearization can perform distortion amplitude and phase matching over a wide frequency range. Generally the distortion amplitude and phase follow a smooth function of frequency.
WIPO Application No. PCT/US2006/023641 (WIPO Application), which is incorporated by reference in its entirety herein, teaches a full quadrant linearizer that can cover wide amplitude and phase range for composite second order distortion (CSO). The WIPO Application discloses in FIG. 2 a linearizer with an in-phase distortion generation function and a quadrature distortion generation. The distortion generator can generate frequency independent second order distortion with positive or negative phase (in-phase) and frequency dependent second order components that have a quadrature phase. A frequency dependent in-phase component can be desired and would require modification to the distortion generator disclosed in FIG. 2 of the WIPO Application. Generation of second and third order distortion can be desired, and can be done with a separate second and third order distortion generation path.
U.S. Pat. No. 5,132,639, ('639 patent) which is incorporated by reference in its entirety herein, teaches generation of second and third order distortion (CTB or composite triple beat) in separate bypass paths that are recombined into a main signal path. The '639 patent discloses a distortion generator in FIG. 6 with one path for second order distortion generation that includes a distortion generator, a filter to adjust the frequency dependence of the distortion output and a delay that is used to adjust the frequency dependence of the distortion phase. These components permit setting any particular amplitude and phase dependence of the generated distortion, but are not flexible in setting variable distortion amplitude and phase as taught in the WIPO Application. The '639 patent also discloses a second path for CTB generation with adjustable amplitude and phase.
U.S. Pat. Nos. 6,574,389 ('389 patent) and 6,593,811 ('811 patent) which are incorporated by reference in their entirety herein, teach in-line distortion generation where the distortion can be generated in the main signal path with a feedback path of amplifiers altered by a non-linear element that modifies the amplifier distortion output. The in-line pre-distortion disclosed in the '389 patent and '811 patent provides a simple implementation to generate distortion but introduces complexity in dealing with device parasitics that affect the main signal gain and distortion generation and limit freedom in adjusting distortion generation amplitude and phase independently of main signal path gain.
The feedback distortion generation as shown in FIG. 2 of the '811 patent can be applied to the emitter path of RF transistors, where the impedance is low and the circuit behavior may not be affected significantly by parasitic capacitance of distortion generating components such as Schottkey diodes. Some implementations can include the use of multiple Schottkey diodes and other distortion generating components (e.g., varactors) and gain adjusting elements (e.g., PIN diodes) all at once at a single emitter node. As a result the feedback networks can be built such that one transistor or gain block can simultaneously generate CSO and CTB and optionally a frequency dependence thereof. As the impedance of the feedback network changes, the gain of the transistor stage changes, which can cause an unwanted variation of the linear signal gain. By adding a variable resistor in the feedback network such as a PIN diode the feedback network impedance can be controlled to obtain or maintain a desired linear gain. One implementation of a distortion generator of this type could be used in the main path intended in the '811 patent. However, main path signal levels are often high, and combining the high power capability of a main path amplifier with a capability to generate variable distortion amplitude and phase and distortion frequency dependence is complex. In another implementation feedback distortion generation in a bypass path permits independent control of distortion amplitude and phase and also frequency dependence using a small number of low power RF transistors.